1. Field of the Invention
The present invention relates to the ammoxidation of saturated hydrocarbons, and more especially, to the conversion of alkanes into a mixture comprising .alpha.,.beta.-unsaturated nitriles.
2. Description of the Prior Art
It is well known to this art that a variety of techniques exist for the ammoxidation of olefins and, in particular, for the ammoxidation of propylene. However, although the saturated hydrocarbons, which are more widely commercially available, are raw materials which are of greater interest from the standpoint of economics, they are also well known not to exhibit a comparable reactivity in this type of reaction for forming, particularly, .alpha.,.beta.-unsaturated nitriles.
One of the difficulties encountered in the ammoxidation of saturated hydrocarbons is the requirement for catalysts capable of dehydrogenating the saturated hydrocarbon under conditions which minimize or suppress the combustion of ammonia and/or that of the hydrocarbon, while at the same time ensuring a reasonable selectivity either for the .alpha.,.beta.-unsaturated nitrile (the desired final product), for example for acrylonitrile from propane, or for final products which are commercially attractive (the aforesaid nitrile and olefin), for example for acrylonitrile and propylene from a propane starting material.
U.S. Pat. No. 3,365,482 describes ammoxidizing isobutane, in particular into methacrylonitrile, on a catalyst based on molybdenum deposited onto eta-alumina, doped with antimony at 508.degree. C. The starting material is a gaseous mixture containing isobutane, air, ammonia and water vapor (1.0/4.5/1.0/12.5); the selectivity for methacrylonitrile attains a value of 49% with an isobutane conversion of 22%.
Starting with a gaseous mixture of propane/air/ammonia/water vapor (1.0/4.7/0.67/12.8) and using the same catalyst at 550.degree. C., the selectivity for acrylonitrile decreases to 15% with a propane conversion of 29%.
French Patent No. 2,027,238 (in part corresponding to U.S. Pat. No. 3,670,009) describes a vapor phase process for the ammoxidation of saturated hydrocarbons at a temperature above 500.degree. C. employing a solid catalyst, notably consisting essentially of tin oxide, boron oxide, molybdenum oxide and silicon oxide. Thus, in Example IX of the Table appearing on pages 12-13 of this '238 patent, the selectivity for acrylonitrile is indicated to be 35%, with a 32% propane conversion, but utilizing operating conditions whereunder the propane/ammonia/air reaction mixture (1/1.2/12) is in the explosive region.
French Patent No. 3,072,334 (corresponding to British Patent No. 1,336,135) describes a vapor phase process for the catalytic ammoxidation of alkanes at a temperature below 500.degree. C. with a high concentration of alkane in the gaseous mixture feedstream, over a solid catalyst of tin oxide and molybdenum oxide (90/10 by weight); however, better results are attained using catalysts of antimony oxide and vanadium oxide.
French Patent No. 2,072,399 describes a process for the catalytic ammoxidation of alkanes, in vapor phase, employing a high concentration of alkane in the gaseous mixture feedstream, over a solid catalyst of, notably, a binary mixture of oxides, including molybdenum oxide.
The following pairs, or couples, are particularly representative:
(Mo, Sb) (Mo, Sn) (Mo, V) (Mo, Ti) (Mo, Bi).
However, none of these pairs performs better than those pairs which do not contain molybdenum. The acrylonitrile yields attained are very low; at best, 1.7% of the propane is converted into acrylonitrile at 570.degree. C. on a catalyst based on tin and titanium oxides.
French Patent No. 2,119,492 (corresponding to U.S. Pat. No. 3,746,737 and to British Patent No. 1,337,759) describes utilizing a binary composition based on molybdenum and cerium oxides. However, the pair (Mo, Ce) performs insufficiently in the absence of halogen or of a halogen compound.
It is also suggested to add to this binary composition (Mo, Ce) a third element selected from tellurium and bismuth (cf. also U.S. Pat. No. 3,833,638). Again, however, the catalyst system appears to perform insufficiently in the absence of halogen or of a halogen compound. Moreover, it too will be appreciated that, in the presence of CH.sub.3 Br the selectivity for acrylonitrile attains a value of 67% with 98% propane conversion, but under operating conditions which mandate a propane/ammonia/air reaction mixture (1/1.2/12) in the explosive region.
French Patent No. 2,119,493 also describes carrying out the vapor phase ammoxidation of alkanes over a solid catalyst containing bismuth and molybdenum oxides and, if appropriate, phosphorus and silica.
Here, too, the catalyst system appears to perform insufficiently in the absence of halogen or of a halogen compound, and the reaction mixture is again in the explosive region.
To obviate the aforesaid numerous shortcomings, various parallel or subsequent investigations have been conducted using solid catalysts based on vanadium and/or antimony.
Thus, Chemistry Letters, pp. 2173-2176 (1989) describes the vapor phase ammoxidation of propane, using multicomponent metal oxides containing molybdenum and bismuth and exhibiting a structure of the scheelite type. It would appear that, despite the relatively moderate temperatures used, the proportion of products of combustion (CO, CO.sub.2 ) is very high in all instances (at least 15%) and that certain catalyst compositions display very little activity in respect of the desired reaction despite being employed under conditions wherein the reaction mixture is in the explosive region or very close to the such region.
The presence of a halogen compound likely will promote corrosion of the apparatus and is therefore not desirable on an industrial scale.
The coproduction of large amounts of CO and of CO.sub.2 is likewise undesirable on an industrial scale.
Lastly, the use of reaction mixtures which are compositionally within the explosive region is all the less desirable on an industrial scale because the process is carried out in a stationary bed.